Advanced Structural Designs

  

ACN 097 789 87    92 Vasey Cres CAMPBELL ACT 2612

Telephone  61612171         Facsimile 61612170

Email   mal@structuraldesigns.com.au

 

 
 
                                                                                                           

                                                             

Using Cold Formed Steel Joists and Bearers to Support your External Timber Decking

 

Due to the increase in the price of timber and the poor durability of some of the cheaper species (Oregon and untreated radiata particularly) many designers are turning to cold formed steel to support timber decking.

 

The advantages of steel over timber are its weight, straightness, dimensional stability and the availability of long lengths.  The disadvantage is mainly the wait if your members are being ordered to size.  Another disadvantage you may not normally think of (or perhaps even notice) is the slight ringing sound you hear when you walk on a deck supported by steel joists.  If you are not sure what we mean, we suggest that you head down to the National Museum of Australia and walk on the decking near the main entrance.  Whilst admiring Mal’s structural designs work on the loop canopy you will also notice a slight ringing as you move along.

 

The issue of long-term durability of this type of construction is often raised and we have seen a number of rusted out balconies in Canberra to testify to the potential for problems.  Steel joists and bearers certainly would be inappropriate in a coastal environment or where any corrosive overflows may occur such as cooling tower supports etc but in Canberra with the appropriate precautions BHP suggest that there should be no problems.  BHP do highlight the need for good ventilation under the deck and we recommend that the bottom of all C purlins have their lip down to avoid catching water as this can lead to early corrosion problems and breed mosquitos.  If you are using top hats as joists a couple of small holes drilled from above into the bottom flanges at mid-span will also help with drainage.  All the above advice relates to BHP sections which have a zinc coating of 350g/m^2.  BHP do produce a 450g/m^2 section  if you are after greater protection.   There are unfortunately a fair number of poorly coated alternatives out there so be careful to inspect coating certificates for whatever arrives on site.

 

You may also need to think again before specifying CCA or tanalith treated decking timbers as these can cause some corrosion problems (talk to us about the termite resistant alternatives).

 

You may have seen published BHP-Lysaght tables covering domestic flooring (TOPFLOOR P10 June 1996) which look very conservative and are not internally consistent.  Bill Barbas, a design engineer with BHP Wollongong urged we to throw these tables in the bin (contacted 14/1/2002). 

 

For bearers we generally recommend C200’s and for joists 120 deep top hat sections.  The table below shows the typical spanning capacity of top hats supporting strip flooring and carrying decking loads.  They are suitable for all residential applications including balcony loading.  If your deck is not a balcony we can design a much cheaper structure than this. 

 

The tables are offered for preliminary design and costing purposes only and we accept no design responsibility or liability unless we certify the final design.  It should be remembered that in addition to the normal design criteria it is important to keep the natural frequency of the floor system above 10 Hz if you are to avoid some of the vibration problems experienced in many cold formed floor designs.

 

Type

Simply Supported

Two Equal Spans

Multiple Equal Spans

TS 61 75

1.35

2.02

1.48

TS 61 10

1.5

1.73

1.65

TS 120 70

2.55

2.9

2.8

TS 120 90

2.8

3.2

3.1

 

If your deck is protected from the elements you could save yourself a few dollars by using purlins instead of top hats.  For typical purlin joist spans see the chart below.

 

Type

Simply Supported

Two Equal Spans

Multiple Equal Spans

C10010

2.5

2.9

2.75

C10012

2.65

3.05

2.9

C10015

2.85

3.3

3.1

C10019

3.1

3.5

3.4

 

Where top hats span the 3.2 metres the tables below show the typical spanning capacity of C 200 purlin sections.

 

Bearer

Simply Supported

Two Equal Spans

Multiple Equal Spans

C20024

3.0

3.0

3.0

 

Although a great many builders are using the Lysaght sections described above we recommend considering the use of the Stramit domestic floor framing system where cold formed joists snap into purpose made back to back bearers.  This may be slightly more expensive for the materials but the construction is far simpler and the overall depth is reduced which can save a couple of brick courses off the building and reduce the steps down to natural ground.

 

If you do not have a height problem and if your joists are gong to be exposed you might like to at also consider the use of  Onesteel’s Duragal flooring system.  You can receive a free booklet on this system by phoning 1800178335.  If you live anywhere close to Sydney you may even find Spantec Systems’s Boxspan a cost competitive alternative Ph (048) 721611.

 

If you have requirements that fall outside the limitations of these charts we suggest you give us a call and we will soon find you a solution. The trick to making this system economical is coming up with quick and neat connection details. 

 

If you need some standard details or want your flooring system certified by an experienced structural engineer in Canberra ACT call Mal Wilson from Advanced Structural Designs on (02) 61612171.  Remember these charts are for preliminary sizing or costing so if you want your layout certified give us a call.

 

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